Applications of solid oxide electrolytic devices include the power generation by SOFCs and production of fuel gases by SOECs. In both devices, SOFCs and SOECs, individual cells are stacked together, with interconnect plates separating the cells, so as to obtain a higher energy output by electricity or by fuel gases, respectively. The interconnect plates separate the fuel gas from the oxidant, which is typically air, and furthermore function as the electrical connection between individual cells in a stack.
Hence, the requirements for an interconnect plate include long term durability, i.e. high oxidation resistance in an oxidizing and reducing atmosphere at high temperatures, i.e. above 500° C., good electrical conductivity in an oxidizing and reducing atmosphere at high temperatures, further a thermal expansion coefficient (TEC) matching with the cell.
Commonly, metallic materials are employed as interconnect materials, since they possess a high thermal and electrical conductivity, are available at low costs and easy to machine.
However, during aging under operation conditions, oxides form on both sides of the metallic interconnect. The growth of said oxides disadvantageously leads to an increased electrical resistance across the interconnect plate and, thus, increased power loss. Therefore, high temperature resistant alloys have been suggested, which contain Si, Al and/or Cr, which form a dense SiO2 (silica), Al2O3 (alumina) or Cr2O3 (chromia) protective oxide layer. Especially alloys forming a chromia layer during operation have been investigated as interconnects due to a good balance of the oxidation kinetics and electrical conductivity of chromia, as compared to silica and alumina. Based on all requirements of the interconnect, ferritic iron-chromium alloys and chromium-rich alloys have so far been considered as the most promising interconnect materials.
U.S. Pat. No. 5,608,174 discloses an oxide dispersion strengthened chromium-rich alloy, having a chromium content of more than 65% by weight. Said alloy forms a chromia scale during aging under operation. The growth rate of chromia at operation temperatures>800° C. is however too high, which in turn results in the electrical resistance across the interconnect plate reaching unacceptable high values due to the low conductivity of chromia.
A further problem when using chromia-forming alloys as interconnects is the evaporation of chromium containing oxides and oxy-hydroxides on the air side of the interconnect during operation. Said evaporation leads to deposition of chromium-containing oxides at the air-electrode-electrolyte interface, which decreases the electrode performance in the long term. This phenomenon is known as “chromium poisoning”.
Attempts to avoid the high electrical resistance and chromium poisoning from the chromia scale have been made by designing alloys which form a duplex Cr2O3— (Mn,Cr)3O4 oxide scale, with the manganese chromium spinel positioned above a layer of chromia.
US-A1-2003/0059335 proposes a chromium oxide forming iron-based alloy, comprising 12 to 28 wt % chromium and small amounts of La, Mn, Ti, Si, and Al. The material is capable of forming at its surface a MnCr2O4 spinel phase at temperatures of 700° C. to 950° C.
EP-B-1298228 relates to a steel material suitable as an interconnect material for fuel cells, the material comprising 15 to 30 wt % Cr and forming oxide films having good electrical conductivity at 700° C. to 950° C.
The formed manganese chromium spinel has advantageously a lower vaporization pressure for chromium containing species than chromia itself, and a higher electrical conductivity. However, the chromium containing spinel still evaporates chromium containing species, and thus a sufficient protection cannot be realized. Moreover, Cr-diffusion is in fact faster in the spinel than in the chromia and thus the formation of a dublex scale leads to an increased rate of the corrosion, thereby reducing the overall lifetime of the device.
It has been further suggested to modify the oxide scale grown on the alloy by applying coatings on the surface of the alloy instead of using the alloy alone. Said coatings may reduce the growth rate of the oxide scale, increase the electrical conductivity of the grown oxide, and reduce the chromium evaporation from the interconnect. The coating of the alloys may, for example, be performed by applying a dense coating on the interconnect, or may be done by applying a porous coating.
U.S. Pat. No. 6,054,231 discloses the application of a metallic coating on the chromium-containing interconnect. The coated interconnect will form a conductive oxide layer containing chromium during aging. The metallic coating is considered to be a sink for chromium diffusing outwards from the alloy.
The proposed coating, however, does not stop chromium containing species from diffusing further outwards from the alloy. Therefore, metallic coatings forming a chromium containing oxide do not act as an effective diffusion barrier towards chromium diffusion. Instead, the metal coating merely impedes the chromium diffusion during the initial stages of the oxidation. Furthermore, the metallic coating does not solve the problem regarding chromium poisoning.
U.S. Pat. No. 5,942,349 proposes to deposit an oxide coating on the interconnect such that a layer of chromium containing spinel is formed in a reaction between a chromia scale formed on the interconnect and the applied oxide coating. The coating initially impedes chromium poisoning of the cathode by catching chromium from the interconnect in the coating forming the spinel.
However, the proposed coating does also not act as a sufficient diffusion barrier for chromium from the interconnect. The oxide layer formed on the interconnect will continue to grow in thickness and thereby result in an increasing electrical resistance across the interconnect plate. Furthermore, Cr-poisoning will occur during long term operation, since the formed spinel becomes itself chromium rich and the respective oxides evaporate therefrom into the air-electrode-electrolyte interface.
Coatings of a similar kind, where a spinel is formed in a reaction between the interconnect and an oxide coating have been proposed in DE-A1-10306649. Said spinel is initially chromium free due to a reaction between the alloy and a spinel forming element in the coating.
However, this coating nevertheless suffers from the above described problems, since the chromium transport from the alloy is not entirely stopped and the reaction layer, although being initially free from chromium, will eventually contain chromium. Thus, Cr-poisoning and increasing electrical resistance will be the result during long term operation. Said coating is, thus, not suitable for applications requiring a very long durability of SOFC and SOEC stacks.
Furthermore, porous coatings of conductive oxides with a perovskite structure have been applied on interconnects as coatings to increase the electrical conductivity of the formed oxide scale and to stop the chromium poisoning, as described in e.g. Y. Larring et al., Journal of the Electrochemical Society 147 (9); 3251-3256 (2000). These coatings have the same drawbacks as mentioned in the above examples.
US-A1-2003/0194592 discloses an interconnect structure for solid oxide electrolytic devices with a coating consisting of two layers. The first layer comprises a Cr-containing electronic conductive oxide covered by a second layer, which acts as a diffusion barrier for oxygen. The second layer also stops chromium diffusion from the first layer. The second layer is a metallic layer, preferably a platinum layer. However, platinum is undesirably expensive, making a commercialization of SOFC and SOEC technology cumbersome.
WO-A1-2006/059942 relates to a strip for use as an electrical contact consisting of a metallic base material which is coated with a metallic layer based on a metal or metal alloy, and further with at least one reactive layer containing at least one element or compound which forms a spinel and/or perovskite structure with the metal or metal alloy when oxidized.
The metal layer coating allows a tailor made perovskite/spinel layer due to a precise control the amount of different elements contained in the metal layer so as to be independent from the composition of the metallic base material. When oxidized, a single perovskite/spinel layer is formed on the metallic base material, which provides a surface with high electrical conductivity and a low contact resistance. Said layer is however insufficient to prevent the further growth of the oxide layer during operation. Furthermore, if a chromium-containing metallic material is employed either as the metallic base material or as a component of the metallic layer, chromium-poisoning will still occur.
WO-A1-2006/059943 discloses a fuel component consisting of a metallic base material coated with at least one metallic layer based on a metal or metal alloy, and at least one reactive layer comprising at least one element or compound which forms at least one complex mixed oxide with the metal or metal alloy when oxidized.
The precise composition of the coating can be tailor-made to achieve the exact formation of the wanted complex metal oxide structure which may be in form of a spinel, perovskite and/or any other ternary or quaternary metal oxide structure upon oxidation with the desired properties, such as good conductivity and good corrosion resistance.
However, the formed oxide layer is insufficient to prevent the further growth of the oxide layer during operation of the fuel component. If furthermore a chromium-containing metallic material is employed as the metallic base material or metallic coating layer, chromium-poisoning will still occur.
The long term durability of the interconnects described in the prior art up to date is not sufficient for many applications. The use of specifically designed alloys for interconnect materials does not eliminate the problem of oxide growth on the interconnect, considerably resulting in an insufficient life time when the interconnects are used in solid oxide cells or the like. Moreover, if chromium-containing metallic materials are employed, which are so far the most preferred materials for interconnects, chromium poisoning of the electrode will still occur; the use of the so far proposed coatings on said alloys cannot eliminate the undesired oxide growth, and does not prevent chromium poisoning. Further, the use of expensive metals, such as platinum, although leading to better results, is not feasible for the commercial potential of solid state devices, such as SOFCs and SOECs, due to the high price.
Alloys utilized for high temperature applications often form a protective silica layer, alumina layer or chromia layer to protect the alloy against further oxidation. Coatings to be applied on alloys to increase the oxidation protection have been suggested in prior art. These include coatings in the ternary phase system Ni—Pt—Al, MCrAlY coatings, TBC coatings, diffusion coatings etc. as described in e.g. J. R. Nicholls, JOM-Journal of the Minerals Metals & Materials Society 52 (1); 28-35 (2000).